firewire: ohci: localize transaction data and rcode per condition branch

The function local variable, transaction_data, in bm_work function is
conditionally used. In the case, the branch-level variable is sometimes
useful.

This commit uses this idea.

Link: https://lore.kernel.org/r/20250908012108.514698-7-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>